feat(markdown): split scroll sync and review comments on the preview

[wagoodman]

Two additions to the markdown source / preview shell.

Split-view scroll sync:

• every rendered block is stamped with the source lines it came from (data-src-line / data-src-end from a markdown-it core rule, on the <pre> for fences), which gives (line, y) anchors between the editor and the preview
• either pane drives the other, interpolating between anchors so tall images and long paragraphs scroll smoothly instead of jumping
• anchors are cached until a re-render or a ResizeObserver hit (late images, mermaid)
• the echo of a programmatic scrollTop write is recognised by the value it lands on, so the panes never feed each other; the pane already on screen leads when split opens, and the first sync waits for attachHiddenScrollRestore so a restored position is not undone

Review comments on rendered text:

• selecting anything in the preview raises a "Comment" pill that opens the same composer as the code side (send now, add to pending, edit, delete)
• the comment goes into the same reviewComments store as an editor comment, so it also shows beside its source in the editor, follows its code through edits, and ships with the pending batch
• the quote is exactly the selection; the line range starts at the innermost block at each end and is narrowed to the source lines the selection spans, matching with whitespace runs elastic since most markdown is hard-wrapped and the rendering joins those lines; when the selection is not in the source as typed (inline markdown), the locator stays the block's lines
• in the preview the quoted text itself is highlighted with a <mark> (not the CSS Custom Highlight API, which mispaints in WKWebView), the open composer's included; the whole-block stripe is only the fallback when the text cannot be found in the rendering
• the composer and card moved out of reviewCommentsExt into commentBoxes.ts (framework-free DOM, .tc-comment-* rules now in index.css) so both surfaces render the same boxes
• in the preview they live in slot divs kept after the top-level block the range ends on, surviving innerHTML rebuilds; find, copy and selection skip them, and copy unwraps the quote highlight
• documents cannot forge the source-line, slot or highlight markers: our line stamps carry a per-load nonce and claimDocument() strips the rest, for the render and each mermaid SVG
• task files only; whole-file comments stay in the editor

Fixes the comment textarea growing 2px short of its text (border-box height set from scrollHeight, which excludes the borders). It overflowed into scrollbars, visible when zoomed, in the editor and diff composers too. It now scrolls only at its height cap.

Screenshots

Selecting text shows a new "comment" button:

Clicking "comment" has the familiar agent comment box:

Not great with a screenshot, but here is the split pane scrolled in the preview, which synced the scroll in the editor:

[simion]

Approved. Nice work on the scroll sync and preview comments, thanks.